There have been recent proposals for implementing the quantum cryptographic protocol BB92 with quantum states based on pulse-position modulation instead of polarization. Attempts to do so to date have been flawed; they are vulnerable to eavesdropping attacks since they do not fully implement the BB92 protocol. Some such approaches are detailed in Nazarathy, “Quantum key distribution over a fiber-optic channel by means of pulse position modulation,” Optics Letters 1533, 30 (2005).
What is needed is a quantum cryptographic protocol BB92 with quantum states based on pulse-position modulation which addresses these issues, and other issues that become apparent in the discussion below.